The present invention relates to cyclohexane derivatives, processes for their preparation and their use as constituents of liquid crystalline materials, especially of dielectrics for liquid crystal display elements.
For liquid crystal display elements, the properties of nematic or nematic-cholesteric liquid crystalline materials are utilized to effect a significant change in their optical properties, such as light transmission, light scattering, double refraction, reflectance or color under the influence of electric fields. The action of such display elements is based, for example, on the phenomenon of dynamic scattering, the deformation of aligned phases (electrically controlled optical birefringence) or the Schadt-Helfrich effect in the twisted cell.
For industrial application of these effects in liquid crystal display elements, liquid crystalline materials are required which have to meet a large number of requirements. Particularly important requirements are their chemical stability towards moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet ranges and electrical continuous and alternating fields. Furthermore, a liquid crystal mesophase in the temperature range from at least 0.degree. C. to +60.degree. C., preferably from -20.degree. C. to +80.degree. C., and a viscosity at room temperature of not more than 70 cP are demanded for liquid crystalline materials which can be used industrially. Finally, these materials must not have any characteristic absorption in the range of visible light, i.e., they must be colorless.
A number of liquid crystalline compounds which meet the stability requirements demanded of dielectrics for electronic components and which are also colorless are already known. These include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628 and the p,p'-disubstituted biphenyl derivatives described in German Offenlegungsschrift No. 2,356,085. In both classes of compounds and also in other known series of compounds having a liquid crystal mesophase, there are no individual compounds which form a liquid crystal nematic mesophase in the required temperature range of from 0.degree. C. to 60.degree. C. Therefore, as a rule, mixtures of two or more compounds are prepared in order to obtain substances which can be used as liquid crystal dielectrics. For this purpose, usually at least one compound having a low melting point and clear point is mixed with another compound having a significantly higher melting point and clear point. This usually gives a mixture which has a melting point at a temperature lower than or about the same as the melting point of the component having a lower melting point, while the clear point is between the clear points of the components.
Components having higher melting points and clear points which have been used hitherto are, for example, 4,4"-disubstituted phenyl p-benzoyloxybenzoates of the formula (II) ##STR2## or the biphenyl ester derivatives of the formula (III) ##STR3## in which formulae R and R' are alkyl or alkoxy and X is carbonyloxy. The compounds of the formula (II) have not, however, found a wide field of application because they impart a high viscosity to the liquid crystal dielectrics containing them. As a result, the switching times of the liquid crystal display elements prepared with these compounds are prolonged in an undesirable manner. It is true that this effect is not observed when the biphenyl esters of formula (III) are used, but these compounds, in particular at low temperatures, are not readily soluble in the most important liquid crystalline base materials, as is desirable if the clear point is to be raised.
Other liquid crystalline compounds are also known, e.g., the phenylcyclohexanes disclosed in Eidenschink et al, Angew. Chem. 89 (1977) 103 and Pohl et al, Phys. Letters 60A (1977) 103. (See also U.S. Pat. No. 4,130,502.) Other cyclohexyl-containing liquid crystalline compounds are disclosed in U.S. Pat. Nos. 4,029,595 and 4,013,582, as well as U.S. patent application Ser. No. 868,580, filed on Jan. 11, 1978 and Ser. No. 868,573, filed on Jan. 11, 1978, now allowed.